1. Field of the Invention
The present invention is generally directed to devices for removably retaining dentures within the mouth of a user and is more particularly directed to improvements in telescopic mountings for removable dental prostheses.
2. State of the Prior Art
It is known in the prior art to make dental prostheses which are removably attachable so as to facilitate cleaning or repair of the prostheses, and to permit ready access to the underlying abutments and gingivae for hygienic purposes and therapeutic aid.
One known method of detachably securing a dental prosthesis makes use of copings fitted onto underlying abutment teeth. The coping is a primary casting having tapered axial outer surfaces which telescopically fits into a retainer socket formed in the underside of the removable prosthesis. The retainer sockets closely conform in shape to the copings and keep the prosthetic appliance seated on the copings during mastication. A refinement on this retention technique takes advantage of precision machine milling methods for refining the axial surface of the coping to a very smooth and accurately tapered shape. Since the mating retainer sockets in the removable prosthesis are produced from an impression taken directly from the machine milled coping, a very close fit between the coping and retainer surfaces is possible, resulting in a precise prosthesis mounting which is rigid yet removable. Such precision fitting of the telescope retainers, i.e. the socket cavities in the prosthesis to the corresponding copings helps produce a positively retained and stable prosthesis mounting.
It is known that the retentive force between the telescoping retainers in the prosthesis and the corresponding copings is a function of the frictional engagement between the axial coping surfaces and the mating retainer surfaces. The frictional engagement in turn is dependent on the angle of taper of the axial coping surfaces. The greater the angle of taper, the lesser the frictional stress between the telescoping surfaces. Conversely, greater frictional retention can be gained by decreasing the angle of taper. As the taper of the axial coping surface approaches zero, i.e. the coping surfaces become parallel and tubular, the retentive force becomes such that the force of removal of the retainer may exceed the bonding strength of the adhesive used to affix the coping to the underlying abutment tooth, with the result that the coping is pulled off the abutment or the tooth fractures in the attempted removal of the prosthesis. It has been determined and known in the literature that the most favorable angle of taper for the axial coping surfaces is six degrees. Copings with a greater taper become too wide at their lower ends, i.e. at their base adjacent to the gums, creating underlying recesses capable of trapping food particles etc., which then produce a hygiene and esthetic problem. Copings with a lesser rate of taper may be excessively retentive. Thus, a compromise between retentiveness and optimum coping size is found at the aforementioned six degree taper.
Another known approach to the removable mounting of dental prosthesis consists of installing a dental post in the root of two or more spaced apart teeth. Such dental posts are fixed in a hole drilled along the root canal of a tooth which has been severed at the gum line, i.e. only the root of the tooth remaining after the crown has been removed. U.S. Pat. Nos. 4,204,321 and 4,290,755 both to Scott and references cited therein disclose dental posts which have an exposed cylindrical stem in which is formed a circumferential groove into which is fitted an elastomeric O-ring. The denture to be secured has a recess configured for receiving the post and also has a peripheral groove in the post receiving recess shaped and sized to closely fit over the O-ring in an interference fit which retains the denture to the post. The O-ring also serves to partially relieve stresses imposed upon the post by the denture during mastication.